We identified the novel protein, Collagen Triple Helix Repeat-Containing-1 (Cthrc1), in a screen for genes associated with vascular injury. It is a unique protein that is highly conserved from lower chordates to mammals, and there are no related molecules listed in the databases. Cthrc1 has an unexpected biogenesis because as it contains a signal peptide for secretion yet remains in the cytoplasm of differentiated smooth muscle cells (SMC). Upon injury Cthrc1, it is found extracellularly where it undergoes proteolytic processing. We predict that cellular trafficking relates to proteolytic processing, because N terminally truncated forms of Cthrc1 are found extracellularly. Overexpression of Cthrc1 leads to profound suppression of interstitial collagen types I and III. Transgenic mice overexpressing Cthrc1 have brittle bones due a lack of collagenous bone matrix, and their arteries are remarkably resistant to neointimal lesion formation upon injury. The antifibrotic effects of Cthrc1 are associated with dramatic inhibition of TGF-ss signaling in SMC. To further assess the role of Cthrc1 in blood vessel formation and arterial remodeling, we have established genetic gain-of-function and loss-of-function mouse strains. In addition, we have generated unique transgenic mouse lines expressing Cre recombinase under the control of the PDGFRss promoter. These lines allow us to control gene expression specifically in the adventitial or medial compartment of blood vessels as well as in pericytes. Approximately 15% of heterozygous Cthrc1mice die during embryogenesis or within one week after birth as the result of severe vascular abnormalities, which include massively dilated vessels affecting small as well as larger vessels. In many tissues of affected mice, bleeding is observed that appears to be caused by rupture of malformed vessels. Thus Cthrc1 is an essential molecule with partial penetrance of lethal haploinsufficiency. The objectives of this proposal are i) to characterize the function of Cthrc1 in blood vessel formation and arterial remodeling, using conditional deletion of Cthrc1 as well as overexpression in the smooth muscle compartment and the adventitia, ii) to characterize the interaction of Cthrc1 with the TGF-ss signaling pathway, and iii) to determine the function of the identified Cthrc1 fragments generated by plasmin. This proposal will employ mouse models of Cthrc1 gain-of-function and loss-of function, in combination with biochemical and cell biological methods to investigate the mechanism of Cthrc1 function.